Drift effects on electromagnetic geodesic acoustic modes

TL;DR

This paper derives a dispersion relation for electromagnetic geodesic acoustic modes (GAMs) considering drift effects, revealing their dependence on electron temperature gradients and explaining observed oscillations in tokamak experiments.

Contribution

It introduces a two-fluid model with parallel viscosity to analyze electromagnetic GAMs with drift effects, highlighting the role of electron temperature gradients.

Findings

Electromagnetic GAM frequency is independent of equilibrium parallel current.

Frequency significantly depends on electron temperature gradient.

Estimated radial wavelength is about 25 cm, larger than typical zonal flows.

Abstract

A two fluid model with parallel viscosity is employed to derive the dispersion relation for electromagnetic geodesic acoustic modes (GAMs) in the presence of drift (diamagnetic) effects. Concerning the influence of the electron dynamics on the high frequency GAM, it is shown that the frequency of the electromagnetic GAM is independent of the equilibrium parallel current but, in contrast with purely electrostatic GAMs, significantly depends on the electron temperature gradient. The electromagnetic GAM may explain the discrepancy between the $f\sim 40$ kHz oscillation observed in TCABR [Yu. K. Kuznetsov $\textit{et al.}$, Nucl. Fusion $\bf{52}$, 063044 (2012)] and the former prediction for the electrostatic GAM frequency. The radial wave length associated with this oscillation, estimated presently from this analytical model, is $\lambda_r\sim 25$ cm, i. e., an order of magnitude higher than the usual value for zonal flows (ZFs).